Optical memory having narrowed track pitch

ABSTRACT

A transparent substrate of an optical memory is produced by injection molding using a stamper. Grooves and lands are alternately arranged on the optical memory, for tracking of light converged on the optical memory. The groove width is set in the range of 0.3 μm to 0.4 μm and the groove depth is set in the rage of 80 nm to 100 nm. A decrease in the width of a land at each edge is restrained to 0.2 μm in maximum. With such dimensions, even when the track pitch is set to about 1.4 μm, it is possible to obtain a track crossing signal with intensity sufficient for performing an access operation to a target track. Moreover, since the dimensions bring about an improved reflectance at a land, the optical memory achieves an improved C/N and recording density.

This is a continuation of application Ser. No. 07/926,224, filed on Aug.5, 1992, abandoned.

FIELD OF THE INVENTION

The present invention relates to an optical memory provided with guidingtracks, on/from which information is optically recorded, reproduced orerased.

BACKGROUND OF THE INVENTION

The development of optical memories on/from which information isoptically recorded, reproduced or erased is in progress. An opticalmemory is a recording medium formed by a substrate in the shape of, forexample, a disk or a card, covered with a recording material film. Inthe case of a disk-shaped substrate, spiral or concentric grooves arepreformed thereon. The grooves and lands between the grooves formtracks. During recording or reproduction of information, a light spotprojected onto an optical memory is controlled to follow the track.

The substrate of the optical memory is produced with the use of astamper 25 manufactured through the processes shown in FIGS. 8(a)through 8(g). Firstly, as illustrated in FIG. 8(b), a photoresist 21 isapplied to the surface of a substrate 20 shown in FIG. 8(a). Secondly,as illustrated in FIG. 8(c), argon laser light 22 is projected ontopredetermined portions of the photoresist 21 so as to record tracks.Subsequently, as shown in FIG. 8(d), the photoresist 21 is developed.Next, a nickel film 23 is formed thereon by, for example, sputtering(see FIG. 8(e)), and a nickel layer 24 is then electroformed on thenickel film 23 (see FIG. 8(f)). Finally, the nickel layer 24 is peeledoff from the substrate, whereby the stamper 25 on which the trackstructure is transferred is obtained as illustrated in FIG. 8(g). Thesubstrate of the optical memory is manufactured by injection moldingusing the stamper 25.

Next, with reference to FIG. 9, the following discusses the structure ofan optical pickup. The optical pickup forms a light spot by converginglight and directs the light spot to follow the track on an optical disk31 as an optical memory. As for the optical disk 31, a recordingmaterial film 39 is formed on the surface of a substrate 30 manufacturedby using the stamper 25.

Light emitted by a semiconductor laser 26 as a light source passesthrough a shaping prism 27 and a first half prism 28, and is thenconverged on the recording material film 39 of the optical disk 31 by anobjective lens 29. Reflected light from the optical disk 31 is reflectedby the first half prism 28 and falls onto a second half prism 32. Afterfalling onto the second half prism 32, the light is separated into two,namely, light directed to a spot lens 36 and light directed to apolarizing beam splitter 33.

The light incident onto the spot lens 36 is directed to pass through acylindrical lens 37 and is then received by a 4-quadrant photodetector38. By detecting the difference between the output signals from the leftand right halves of the 4-quadrant photodetector 38, a tracking errorsignal is obtained. The tracking error signal indicates deviation of thelight spot from the track center.

Since the light spot is diffracted by the track when traversing thetrack, the amount of reflected light changes. A track crossing signal isderived from the sum of the output signals from the four detectionsections of the 4-quadrant photodetector 38. Namely, the track crossingsignal indicates changes in the amount of reflected light when the lightspot traverses tracks one after another. When the optical pickup movesto access to a target track, the number of tracks traversed by the lightspot is detected by counting the number of times the waveform of thetrack crossing signal reaches its positive peak value. Then, the opticalpickup is positioned according to the detected number.

Meanwhile, the light incident onto the polarizing beam splitter 33 isfurther separated into two and received by photodetectors 34 and 35,respectively. Accordingly, various other signals are generated.

FIGS. 10(a) and 10(b) illustrate a relation between a push-pull signal(tracking error signal) and a track crossing signal. FIGS. 10(a) and10(b) indicate the push-pull signal and the track crossing signalproduced when the optical pickup moves in a direction and crosses thetrack, respectively.

There is a phase difference of 90 degrees between the track crossingsignal and the push-pull signal, and the phase of the track crossingsignal is delayed by 90 degrees with respect to that of the push-pullsignal as shown in FIGS. 10(a) and 10(b). The phase difference betweenthe track crossing signal and the push-pull signal varies depending onthe moving direction of the optical pickup. If the optical pickup movesin the opposite direction, the phase relation between the track crossingsignal and the push-pull signal is inverted and the track crossingsignal advances by 90 degrees with respect to the push-pull signal.Thus, the moving direction of the optical pickup is detected bydetecting the phase relation between the track crossing signal and thepush-pull signal. This makes it easier for the optical pickup to accessto a target track.

The track crossing signal and tracking error signal thus obtained varygreatly depending on track parameters, such as the width and depth ofgrooves forming the tracks and the track pitch. Since the C/N of thesignals improves when the reflectance at the track increases, it isdesirable to make the reflectance as high as possible.

Moreover, to enhance the recording capacity of the optical memory,recording density must be increased. One of the effective methods toincrease the recording density of the optical memory is that increasingthe recording density along the track direction while decreasing thetrack pitch. However, since the track parameters change when the trackpitch is decreased, the respective signals derived from the tracks varysignificantly. Thus, in order to obtain appropriate signals andreflectance, the track parameters must be designed carefully.

As shown in FIG. 11, the intensity of the track crossing signals wasmeasured with respect to various groove depths and widths. Thesemeasurements were carried out using optical memories with a track pitchof about 1.6 μm, an objective lens with a numerical aperture (NA) of0.55, and laser light with an 830-nm wavelength. The intensity of thetrack crossing signal is normalized on the basis of the intensity oflight reflected from a non-groove portion.

It can be seen from FIG. 11 that the intensity of the track crossingsignal becomes maximal when the width of a groove is 0.3 μm to 0.4 μm.Moreover, in the case where the depth of a groove is in the range of 70nm to 100 nm, the intensity becomes greater as the depth of the groovebecomes greater. The sufficient intensity of the track crossing signalvaries depending on each system. A groove depth of about 70 nm or moreis required in order to obtain a sufficient intensity of, for example,about 0.2 in each system. Thus, when the track pitch is about 1.6 μm,the appropriate width and depth of the groove are about 0.35 μm and 70nm, respectively.

However, if grooves are formed with the above-mentioned dimensions butwith a smaller track pitch, the results shown in FIG. 11 are unlikelyexpected.

The following document discusses track parameters in detail. The titleof the document is "Designing preformed grooves and preformed pits ofoptimum dimensions for push-pull/tracking servo system", Optical MemorySymposium '90, p. 11. However, this document describes only the trackparameters when the track pitch is 1.6 μm and does not mention designingof track parameters with respect to a smaller track pitch.

The dimensions of tracks are described in the following documents."Optical pregroove dimensions: design considerations", Applied Optics,Vol. 25, No. 22, Nov. 15, 1985, p. 4031; Japanese Publication forUnexamined Patent Application No. 100248/1983, No. 102347/1983, No.102338/1983, No. 38943/1984, No. 38944/1984 and No. 11551/1984; andJapanese Publication for Unexamined Utility Model Application No.165794/1983. However, these documents do not discuss track pitch.

The cases where a smaller track pitch is used are described in thefollowing documents. "Magneto-optical disk by contact printing method",SPIE Vol. 1078, Optical Data Storage Topical Meeting, 1989, p. 204; and"High-density magneto-optical disk using a glass substrate", PapersPresented at Kansai District Regular Science Lecture Meeting of Societyof Precision Optics, 1988, p. 107. However, since optimization of trackparameters was not performed according to those documents, some problemsarise. Namely, the access operation can not be performed with accuracy,because the level of the track crossing signal is lowered when the trackpitch becomes smaller.

Next, problems associated with the manufacturing methods of the stamper25 of FIG. 8 are discussed below. The light spot of the argon laserlight 22, which is used when manufacturing the stamper 25, normally hasGaussian distribution or a similar intensity distribution. Namely, adistribution where the light intensity continuously decreases from thecenter of the light spot outward and it decreases gradually, inparticular, at the circumferential section of the light spot.

Therefore, when recording is performed with the argon laser light 22having such an intensity distribution, the developed photoresist 21 hascurved edges. As a result, when substrates for optical memories areproduced through injection molding using such a stamper 25, the edges ofthe lands on these substrates are curved because of the intensitydistribution of the argon laser light 22.

Thus, during recording or reproducing of information, when the opticalpickup projects a light spot onto the optical memory whose lands havenarrowed flat portions due to the curved land edges, the reflectance atthe tracks on the optical memory decreases. Consequently, the quality ofthe signals deteriorates.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical memoryhaving optimum track dimensions which ensure appropriate track crossingsignals and reflectance even when the track pitch is narrowed so as toimprove recording density.

In order to achieve the above-mentioned objective, an optical memory ofthe present invention incorporates a substrate provided with tracksformed by arranging grooves and lands alternately. The grooves and landsare provided so as to perform tracking of light when it is converged onthe optical memory. The grooves and lands are formed such that thegroove width is in the range of 0.3 μm to 0.4 μm, the groove depth is inthe range of 80 nm to 100 nm, and that the maximum limit for a decreasein the width of the land at each edge is 0.2 μm.

By setting the groove width and the groove depth within these ranges,even when the track pitch is narrowed to about 1.4 μm, it is possible toobtain a track crossing signal with intensity sufficient for performingan accurate access operation to a target track. Moreover, since thedecrease in the flat portion of the land is restrained, the reflectanceat the land increases, achieving satisfactory signal quality.

To restrain the decrease in the flat portion of the land, it isdesirable to form the grooves by etching a master substrate directlyduring manufacturing of the master substrate.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a graph illustrating the profile of the substrate of anoptical memory of the present invention, and FIG. 1(b) is a graphillustrating that of an optical memory of a conventional type.

FIG. 2 is an enlarged perspective view of part of an optical memoryhaving the profile shown in FIG. 1(a).

FIGS. 3(a) through 3(i) are depictions illustrating the processes ofmanufacturing a stamper used for producing the optical memory of thepresent invention.

FIG. 4 is a graph illustrating the relation between the groove width anddepth and the intensity of a track crossing signal when the track pitchis about 1.4 μm.

FIG. 5 is a graph illustrating the relation between the reflectance atthe land and groove and the groove depth.

FIG. 6 is a graph illustrating the relation between the groove width anddepth and the C/N.

FIG. 7 is an enlarged vertical sectional view of the optical memory, andthe results shown in FIG. 6 were obtained by using this optical memory.

FIGS. 8(a) through 8(g) are depictions illustrating the processes ofmanufacturing a stamper used for producing a conventional opticalmemory.

FIG. 9 is a depiction illustrating the structure of a conventionaloptical pickup.

FIGS. 10(a) and 10(b) are graphs illustrating the waveforms of apush-pull signal and a track crossing signal obtained by the opticalpickup of FIG. 9.

FIG. 11 is a graph illustrating the relation between the groove widthand depth and the intensity of a track crossing signal when the trackpitch is about 1.6 μm.

DESCRIPTION OF THE EMBODIMENT

With reference to FIGS. 1 through 7, an embodiment of the presentinvention is described below.

As illustrated in FIG. 2, when manufacturing an optical memory 15according to this embodiment, spiral or concentric grooves 2 are formedon the surface of a transparent substrate 1 of the optical memory 15.For example, the transparent substrate 1 is made from a polycarbonateresin or an amorphous polyolefine resin. In this embodiment, a land 3formed between two adjacent grooves 2 is used as a recording track. Thetrack pitch 7 is given by adding the width of a single groove 2 and thatof a land 3. Instead of a conventional track pitch of about 1.6 μm, inthis embodiment the track pitch 7 is set to about 1.4 μm.

Laser light 4 emitted by a light source, not shown, is directed to theoptical memory with the above-mentioned dimensions and is converged intoa light spot 6 on the land 3 by an objective lens 5. It is also possibleto converge the light spot 6 on the groove 2 if the groove 2 is used asa recording track, instead of the land 3. In this case, since therelation between the groove 2 and the land 3, to be described later, isunchanged, similar results are expected.

The numerical aperture (NA) of the objective lens 5 is set to about 0.5to 0.55, and the wavelength of the laser light 4 is set in the rangefrom 780 nm to 830 nm. An excessively great NA causes increases in thesize and the weight of the objective lens 5, and tracking of the lightspot 6 to be affected by the tilts of the transparent substrate 1 andthe objective lens 5. Therefore, in order to make the light spot 6accurately follow the recording track on the optical memory 15 by usingthe objective lens 5 with a large NA, the system must satisfy stricterrequirements with respect to mechanical precision. Consequently, themanufacturing cost of the system increases.

Regarding the wavelength of the laser light 4, a shorter wavelength ismore suitable for high-density recording. However, in order to obtainlaser light of a short wavelength, a large gas laser or a semiconductorlaser which is expensive and hard to obtain is required. On the otherhand, if the NA of the objective lens 5 and the wavelength of the laserlight 4 are in the above-mentioned ranges, it is possible to use asemiconductor laser and an objective lens 5 which are compact, lessexpensive, and easy to obtain. This makes it possible to constitute asystem which is less expensive overall.

For the manufacturing of the transparent substrate 1, for example, astamper 14 of FIG. 3(i) is used. With reference to FIGS. 3(a) through3(i), the following explains the manufacturing processes of the stamper14.

Firstly, as illustrated in FIG. 3(b), a photoresist 9 is applied to thesurface of a quarts substrate 8 shown in FIG. 3(a). Secondly, asillustrated in FIG. 3(c), argon laser light 10 is applied topredetermined portions on the photoresist 9 so as to record recordingtracks and control signals necessary for accessing to target tracks. Thecontrol signals are recorded in the form of pits.

Then, the photoresist 9 is developed as shown in FIG. 3(d).Subsequently, as illustrated in FIG. 3(e), portions, which are notcovered with the photoresist 9 and thus where the quarts substrate 8 isexposed, are etched with the dry etching method. A gas composed mainlyof a fluorine compound such as carbon tetrafluoride (CF₄) is used foretching. Through the etching process, grooves 11 are formed directly onthe quarts substrate 8. This process is controlled so as to make thewidth and depth of the grooves 11 within predetermined ranges, to bedescribed later. Moreover, as to be described later, this etchingprocess restrains the edges of the lands 3 formed on the transparentsubstrate 1 from being curved.

Next, as illustrated in FIG. 3(f), the photoresist 9 is removed from thequarts substrate 8. Then, a nickel film 12 is formed on the surface ofthe quarts substrate 8, for example, by sputtering as shown in FIG.3(g). Subsequently, a nickel layer 13 is electroformed as shown in FIG.3(h). Finally, as illustrated in FIG. 3(i), by peeling off the nickellayer 13 from the quarts substrate 8, the stamper 14 having thereon thenegatively transferred grooves and lands is obtained. And, thetransparent substrate 1 shown in FIG. 2 is produced by injection moldingusing the stamper 14.

Also formed on the surface of the transparent substrate 1 is a recordinglayer (not shown) on/from which information is optically recorded,reproduced or erased. As for a material for the recording layer, forexample, the following are listed: a magneto-optical recording materialusing magneto-optical effects; a phase-change material using a phasechange between crystal and amorphous; a write-once material using pitsformed by recording energy; and a recording material using photochromiceffects.

As illustrated in FIG. 7, this embodiment uses a recording layer ofmagneto-optical type. The magneto-optical recording layer is composed offour films, AlN 16, DyFeCo 17, AlN 18 and Al 19, formed in this order onthe substrate 1 by sputtering. The thickness of each of the films 16-19,is 80 nm, 20 nm, 20 nm, and 30 nm, respectively. The DyFeCo 17 is anamorphous alloy film with a composition of Dy₂₃.5 (Fe₇₈ Co₂₂)₇₆.5.

Various optical memories having a track pitch of about 1.4 μm anddifferent groove depths and groove widths were produced. Experimentswere carried out to measure the intensity of the track crossing signalsderived from these optical memories, and the results are shown in FIG.4.

In FIG. 4, the horizontal axis represents the widths of the grooves,while the vertical axis indicates the intensity of the track crossingsignals which is normalized on the basis of the intensity of a trackcrossing signal derived by projecting light onto a flat portion havingno groove. As is seen from FIG. 4, the track crossing signal with themaximum intensity is obtained when the groove width is in the range of0.3 μm to 0.4 μm.

Besides, if a comparison is made between the intensity of a trackcrossing signal from the optical memory with the conventional trackpitch of about 1.6 μm (see FIG. 11) and that from the optical memorywith the track pitch of about 1.4 μm, it can be seen that the intensityof the track crossing signal is significantly lowered as a whole whenthe track pitch is 1.4 μm. When the track pitch is about 1.4 μm and thegroove depth is 70 nm, the intensity of the track crossing signal isparticularly lowered to have the maximum intensity of about 0.12. Toperform an accurate access operation to a target track, such intensityis insufficient.

Therefore, when the track pitch is made about 1.4 μm, it is notappropriate to form grooves with dimensions similar to the dimensionsdesigned for the track pitch of about 1.6 μm. Accordingly, in order toobtain a track crossing signal with sufficient intensity, the groovedepth must be made greater than 80 nm.

Next, optical memories having a groove width of about 0.35 μm, a trackpitch of about 1.4 μm and different groove depths were produced throughthe processes described above. Reflectance with respect to these opticalmemories was measured, and the results are shown in FIG. 5. The solidline and the dotted line in the drawing indicate the reflectance (A) ata land and the reflectance (C) at a groove, respectively.

Also, using the stamper 25 manufactured through the conventionalprocesses shown in FIG. 8, an optical memory with a groove width ofabout 0.35 μm, a track pitch of about 1.4 μm and a groove depth of about85 nm was produced. Then, its reflectance was measured. ∘ in FIG. 5shows the reflectance (B) at a land of this conventional-type opticalmemory, while  indicates the reflectance (D) at a groove.

The amplitude of the track crossing signal is correlated with thedifference between the reflectance at the land and the reflectance atthe groove. FIG. 5 indicates that, in both the optical memory of thisembodiment and the conventional-type optical memory, the differencebetween the reflectance at a land and the reflectance at a groove isabout 0.2. Namely, there is not much difference in the amplitudes of thetrack crossing signals between these optical memories. However, when thegroove depth is 85 nm, the reflectance at a land is about 0.7 in thecase of the conventional-type optical memory, while it becomes higher toabout 0.8 with respect to the optical memory of this embodiment. Withthe present invention, as described above, since the reflectance at theland becomes higher, the C/N ratio improves, resulting in high-qualitysignals.

FIG. 6 illustrates the relation between the C/N and the depth and widthof grooves. As a result of the relation between the reflectance and thegroove depth shown in FIG. 5, the C/N improves as the depth of groovedecreases. It can also be seen that the C/N improves as the width ofgroove decreases or as the width of land increases.

The reasons for the improvement of the reflectance at the land isdiscussed below. FIG. 1(a) shows the profile of the grooves of theoptical memory of this embodiment, measured by an STM (scanning typetunnel microscope). FIG. 1(b) illustrates the profile of the grooves ofthe conventional-type optical memory, measured by the STM.

As illustrated in FIG. 1(b), the degree of curving of the edges of theland is expressed by a distance d between X and Y, where X is theintersection point of the extended oblique line of the groove and theline extended from the top of the land and Y is the contact point of thetop and the edge. As the distance d increases, the edges of the landbecome more rounded and the flat portion thereof decreases. In the caseof the conventional-type optical memory, the distance d is about 0.2 μm.On the other hand, on the optical memory of the present invention, sincethe edges of the land are not rounded much, a decrease in the flatportion of the land is very little.

As described above, the roundness of the edges of the land is greatlyowing to the intensity distribution of the argon laser light used in theproduction of the stamper. Therefore, as illustrated in FIG. 3, astamper 14 is produced using the quarts substrate 8 on which grooves aredirectly etched. Then, as shown in FIG. 1(a), by performing injectionmolding, a substrate having lands with scarcely reduced flat portions isobtained.

It is found from these results that in order to achieve a satisfactoryC/N with a highest possible reflectance at lands, a decrease in thewidth of the optical memory's land at each edge must be restrainedwithin 0.2 μm.

Considering the above-mentioned results, it is possible to achieve atrack crossing signal with sufficient intensity as well as asatisfactory C/N by designing a groove having a width in the range of0.3 μm to 0.4 μm and a depth in the range of 80 nm to 100 nm and bysetting the maximum limit for a decrease in the width of a land at eachedge to 0.2 μm. Since these results are not disclosed in any of theabove-mentioned publications, these are considered to be novel designingindexes in manufacturing of an optical memory with a smaller trackpitch.

Moreover, regarding compact disks and video disks, the track pitch forthese disks is usually set to 1.6 μm. However, if the track pitch is setto around 1.4 μm, the track density increases by around 14%, resultingin an improved recording density.

As described above, by setting track parameters based on the designingindexes newly established by the present invention, it is possible tomanufacture an optical memory with: (1) an improved recording density;(2) a track crossing signal whose intensity is sufficient for performingan accurate access operation to a target track; and (3) a satisfactoryC/N.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. An optical memory, comprising a substrate havingthereon tracks formed by alternately arranged grooves and lands, saidgrooves and lands being provided so as to perform tracking of lightconverged on said optical memory, wherein, for a track pitch of lessthan 1.6 μm, a groove width is set in the range of 0.3 μm to 0.4 μm, agroove depth is set in the range of 80 nm to 100 nm, and the maximumlimit for a decrease in the width of said land at each edge is set to0.2 μm, said groove width, groove depth and maximum limit being set tomaximize the strength of the track crossing signal and to improve theC/N ratio of said memory.
 2. The optical memory according to claim 1,wherein said grooves are formed with a track pitch of 1.4 μm.
 3. Theoptical memory according to claim 1, further comprising a recordinglayer on or from which information is optically recorded, reproduced anderased, said recording layer being formed on said substrate.
 4. Theoptical memory according to claim 3, wherein said recording layer ismade from a magneto-optical material, and wherein information isrecorded, reproduced and erased on or from said recording layer by usingthe magneto-optical effects of said magneto-optical material.
 5. Theoptical memory according to claim 3, wherein said recording layer ismade from a phase-change material, and wherein information is recorded,reproduced and erased on or from said recording layer by using a phasechange of said phase-change material between crystal and amorphous. 6.The optical memory according to claim 3, wherein said recording layer ismade from a write-once type material, and wherein information isrecorded, reproduced and erased on or from said recording layer by usingpits formed by recording energy.
 7. The optical memory according toclaim 3, wherein said recording layer is made from a recording material,and wherein information is recorded, reproduced and erased on or fromsaid recording layer by using the photochromic effects of said recordingmaterial.